1. Field of the Invention
The present invention relates to a method of controlling the time constant of a filter and a filter circuit having the time constant control function based on the method, and more particularly to a time constant control method suitable for a filter of a receiving unit of a paging radio receiver which is required to be reduced in voltage, power and size.
2. Description of the Related Art
In a conventional filter time constant control system, the operation of filtering a received signal and the operation of controlling the time constant of a filter circuit for filtering are carried out in parallel.
For example, there are provided a filter for filtering an input signal (hereinafter referred to as a main filter), a control circuit for controlling the time constant of the main filter, and an artificial filter having the same construction as that of the main filter, whereby a standard pulse signal having a predetermined frequency is applied or inputted to the artificial filter so that the artificial filter may deliver an output signal the level of which is compared with a predetermined reference level. As a result of the comparison, a voltage or current complying with an error between the output and reference levels is fed back to the artificial filter and supplied to the main filter, in a polarity which cancels the error.
In the above arrangement, when the relative accuracy (inclusive of control characteristics) between the main and artificial filters is designed to be high, the voltage or current fed back to the artificial filter can act on the main filter so as to cancel the error in the main filter.
Incidentally, used as the time constant control element within the filter is, for example, "ON" resistance of variable capacitor diodes or D-MOS's which are controllable by voltage or "ON" resistance of diodes which are controllable by current; but capacitors and switches acting as control elements which are on/off controlled for connection of capacitors are not used. The reason for this is as follows. As described previously, in the conventional receiving system, filtering of the received signal and time constant control of the main filter are carried out in parallel and therefore, if the control element such as the capacitor is used as the time constant control element, then the switch will be turned on or off, causing a noise which interferes with the main filter.
In another conventional system, an oscillator is constructed using time constant control elements within an artificial filter, a phase difference between an oscillation signal from the oscillator and a standard pulse signal having a predetermined frequency is detected, and a voltage complying with the phase difference is supplied to the artificial filter and a main filter. The voltage complying with the phase difference acts on the main filter as it acts on the artificial filter.
The aforementioned conventional systems are disclosed in JP-A-60-214617 and "On-chip Automatic Tuning for a CMOS Continuous-Time Filter", by Mihai Banu et al., ISSCC85/FRIDAY, FEBRUARY 15, IEEE International Solid-State Circuits Conference.
There is a desire to make a filter in the form of an LSI with its size and power reduced and especially to realize a small-sized paging radio receiver of reduced power consumption by using the filter in the form of the LSI. However, a ceramic filter is widely used in the conventional paging radio receiver and the size reduction of the receiver is limited. Further, in realizing the filter portion with the LSI, the monolithic (main) filter using variable capacitor diodes as time constant control elements must be used in combination with the artificial filter of the same construction and control circuit which are adapted to control the main filter, as described previously. Accordingly, the total area of the filter circuit inclusive of the control circuit and artificial filter causes a bottleneck to size reduction. In addition, the time constant control is effected during the signal receiving operation and disadvantageously power consumption is increased correspondingly.
Specifically, when the variable capacitor diode is used as the time constant control element of the filter, a control voltage of about 3 V is required for controlling the capacitance of the filter by about .+-.25%. When the time constant control element is so designed that "ON" resistance of D-MOS transistors is controlled by a gate voltage, the filter must be a differential active filter in order to compensate harmonic distortion generated by non-linearity of the D-MOS transistors and the use of the differential active filter is unsuited for redution of the circuit area.